1. Field of the Invention
This invention relates to an image-transmitting optical fiber for use in a remote-control system of emission spectroscopic analysis, fluorescence spectroscopic analysis and absorption spectroscopic analysis.
2. Prior Art
FIG. 1 schematically shows an emission spectroscopic analysis system. A plasma generator 10 comprises positive electrodes 12 and a negative electrode 13, and a sample 14 is introduced into a plasma flame 16 produced by the positive and negative electrodes 12 and 13, so that the sample 14 is heated and excited to cause emission, the emission portion being designated by numeral 18 as best shown in FIG. 2. An image-transmitting fiber 20 extends into a housing 22 of the plasma generator 10 and has a condenser 24 secured to one end thereof in the housing 22, the image-transmitting fiber 20 comprising a plurality of fiber elements. The one end of the image-transmitting fiber 20 is fixed relative to a device 26 for the fine adjustment of the optical axis of the fiber 20. The image-transmitting fiber 20 extends through a spiral hole 28a in a block 28 mounted in a wall 30 of concrete. First, an ocular lens 32 is attached to the other end of the image-transmitting fiber 20 remote from the condenser 24, and the condenser 24 is brought into alignment with the emission portion 18 by the optical axis adjustment device 26. Then, the ocular lens 32 is removed from the other end of the image-transmitting fiber 20, and the other end is connected to a housing 34 of a spectroscopic analysis device 36 so that the image of the emission portion 18 is transmitted thereinto through the image-transmitting fiber 20 and a slit 34a of the housing 34. As best shown in FIG. 2, the emission portion 18 is enclosed by the plasma 12 of an inverted Y-shape.
The image-transmitting fiber 20 serves as an image sensor for viewing the emission portion 18 and transmitting its image. Also, the image-transmitting fiber 20 serves to transmit the power of the light of the emission portion 18. Since the image-transmitting fiber 20 performs the function of the image sensor, it must have a high resolution. To achieve this, it is desired to increase the number of the fiber elements of the image-transmitting fiber 20 which elements serve to transmit the image of the emission portion 18. However, it is undesirable that the image-transmitting fiber 20 becomes too large in diameter, because the flexibility of the fiber 20 is affected. Therefore, when it is desired to increase the number of the fiber elements while limiting the diameter of the image-transmitting fiber 20, each of the fiber elements has to be small in diameter, each fiber element being composed of a core and a cladding around the core. As a result, the amount of the energy leaking into the cladding becomes larger. Generally, the core of each fiber element of an image-transmitting fiber is made of pure silica while the cladding is made of fluorine-doped silica. The wavelength measured in spectroscopic analysis is between ultraviolet and visible light range. The cladding of fluorine-doped silica exhibits a greater loss, that is, poor initial characteristics, particularly at a short wavelength range of 0.24 to 0.35 .mu.m. Therefore, when the energy leaks into the cladding, a transmission loss of light power becomes considerably high. In addition, the cladding of fluorine-doped silica possesses extremely poor radiation-resistance characteristics in comparison with pure silica. Therefore, when the image-transmitting fiber of the type described is used in an environment in which it is subjected to radioactivity, the transmission loss further becomes greater.
The amount of leaking of the energy into the cladding depends largely on the diameter of the core of the fiber element. More specifically, the greater the core diameter becomes, the amount of leaking of the energy becomes less to thereby enhance the characteristics of the fiber element. Therefore, a simple solution is to increase the diameter of the core of the fiber element. This approach is disadvantageous, however, in that the diameter of the fiber element becomes greater, so that the overall diameter of the image-transmitting fiber becomes greater. The image-transmitting fiber of such a diameter is less flexible, and hence can not be bent with a smaller radius, and can not be handled easily.
In order to keep the transmission loss due to the cladding of the fiber element to an acceptable level, it is necessary that the core of the fiber element should have a diameter of about 50 .mu.m. However, it is impractical to provide the image-transmitting fiber composed of fiber elements having such a core diameter.
As described above, the image-transmitting function and light power-transmitting function of the fiber element are incompatible, and if it is desired to improve one of them, then the other becomes worse.